Fix FloatingToolbar flicker in reaction to text cursor blink.

1. Reposition the toolbar on predraw only when positioning has changed.
2. Update the toolbar popup's position only if the content rect changed.
3. Fix FloatingToolbarPopup.cancelOverflowAnimations().
   The previous implementation wasn't actually cancelling the animation.

(1) is enough to fix the bug. But (2) and (3) fix issues in the
toolbar directly related to this bug.

Bug: 22039189
Change-Id: I84ec793d788f9402a1f8635e68e3344746f6af07